Device comprising electric light source fed by pulses



A ril 1, 1958 I P. HOEKSTRA ,8

DEVICE COMPRISING ELECTRIC LIGHT SOURCEFED BY PULSES Filed March 28, 1955 INVENTOR PIET HOEKSTRA United States Patent DEVICE COMPRISING ELECTRIC LIGHT SOURCE FED BY PULSES Piet Hoekstra, Eindhoven, Netherlands, assignor, by

mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application March 28, 1955, Serial No. 497,337

Claims priority, application Netherlands April 9, 1954 3 Claims. (Cl. 315-208) This invention relates to a device for producing a light flash for a predetermined period of time. More particularly, the invention relates to a device comprising an electric light source which is fed by pulses via a controlled gas-filled or vapor-filled relay tube for obtaining one or more light flashes. Such a device may be used, for example, in a film projector in which the projector lamp emits one or more light flashes during each image of the film. The relay tube is controlled by means of an electric pulse on the grid in such manner that the tube becomes conducting during the flashing time, whereafter the current in the normal operating condition is interrupted. The resulting current pulse also passes through the projector lamp, causing it to emit a light flash. This process may be repeated at a frequency which is determined by that of the pulses.

It has been found that in certain cases the circuit is not interrupted due to the fact that the relay tube, after the end of the flashing period, remains conducting and an abnormal operating condition arises in which a continuous current flows and the effect of the use of light flashes disappears. It will be evident that more particularly in the case of film projectors the operation is thus rendered impossible, since the use of light flashes in this domain would have to be abandoned and the advantages resulting from this use, for example longer period of life of the light source and omission of the shutters, are thus lost.

In order to avoid the disadvantages resulting from the aforementioned abnormal operating condition, in a device according to the invention the circuit of the relay tube and of the light source includes a switch. The switch may be operated by means of a control voltage which is derived from an impedance included in the circuit. The control voltage acts upon the switch after the device has been switched into the normal operating condition, the relay tube then being conducting, in such manner that the cir cuit is closed. If the relay tube is conducting after the end of the flashing period a control voltage ensues such that the circuit is interrupted by the switch.

In one particular embodiment of the invention, the circuit is interrupted by means of a mechanical switch operated by a relay supplied via a controlled rectifier from an alternating voltage source. The control voltage is supplied to the control member; the relay being of a design such that the current passing through the relay keeps the switch closed in the position in which the relay tube is cut ofl. The control voltage in thenormal operating condition decreases the mean current through the relay to such an extent only that the switch keeps the circuit closed, while the mean relay current in the event of the relay tube remaining conducting, is decreased by the control voltage to such an extent that the switch interrupts the circuit.

In order that the invention may be readily carried into eflect, it will now be described, by way of example, with reference to the accompanying drawing wherein the single figure is a schematic diagram of an embodiment of the device of the present invention.

2,829,315 Patented Apr. 1, 1958 conducting. Such a discharge results in a damped LC-' oscillation bringing about a flash current in the projector lamp. Since during the subsequent half-wave of the oscillation, the polarity of the voltage across the relay tube 2 is reversed, the tube 2 is cut olf and thus prevents further oscillating. This process may be repeated due to the relay tube being again rendered conducting after a time interval by means of a pulse on the grid 7, so that the capacitor 5, which has been charged in the meantime, can again produce a damped LC-oscillation, leading to the production of a single flash. In the normal operating position, in the event of a plurality of flashes being produced, the relay tube is allowed to be conducting only during the flashing periods, the tube having to be cut oil during the remaining part of thetime.

As described above, after the circuit has been switched on, there is the risk of a continuous current passing through the relay tube 2. In this case the capacitor 5 does not charge further and the lamp 1 is fed directly from the source 3 via the relay tube 2. This condition is stable, since the damped LC-oscillation does not occur and there is no cause for cutting ofi the relay tube 2.

According to the present invention, the circuit of tube 2 and. lamp 1 includes the mechanical switch 13 which may be operated by means of a control voltage derived from a resistor 9 included in the circuit in series with the lamp 1. A relay 8 is fed from the alternating current source by way of a transformer 12 via a controlled rectifier comprising two gas-filled triodes 10 and 11, the control via the interconnected grids of the triodes 10 and 11 being effected by means of the control voltage provided by resistor 9. The supply circuit of the relay 8 also includes a mechanical switch 14. The relay 8 is of a design such that, when the switch 14 is closed, a current passing through said relay can operate the switch 13, resulting in the circuit being closed. When a pulse on the grid 7 of the tube 2 produces a current pulse, then in the normal operating position, in which the light flashes are of a very short duration, a resultant control voltage which is also pulsatory reduces the mean current through the relay to such an extent only that the switch 13 remains closed and the normal operating condition is maintained. The decrease in current through the relay 8 results from the fact that, during the flashing period, the triodes 10 and 11 are cut off by the control voltage, so that the supply of the relay is discontinued (during this period). This does not much affect the total average time due to the particular design of the relay and the short duration of the flash period. However, when in the aforementioned abnormal operating condition an abnormal current occurs due to the relay tube 2 remaining conducting, the resultant control voltage decreases the mean current through the relay 8 to such an extent that the switch 13 is de-energized and interrupts the circuit. Consequently, the control voltage disappears, the switch 13 is re-energized and the flashing process can start again.

' This may be repeated until the normal operating position me als 3 What is claimed is:

1. A device comprising an electric light source for producing a light flash for a predetermined period of time, circuit means for supplying pulses to said light source to energize same, said circuit means comprising a controlled discharge tube having an ionizable medium, a switch and an impedance, means for deriving a control voltage from said impedance and means controlled by said control voltage for actuating said switch to deenergize said light source when said light source is energized for a period of time longer than said predetermined period of time.

2. A device as set forth in claim 1, wherein said switch is a mechanical switch and wherein said means for actuating said switch includes a controlled rectifier having a control member, a relay, and means for supplying energy to said relay via said controlled rectifier, said control voltage being applied to the control member of said rectifier.

3. A device comprising a capacitor, a first series circuit including a direct current source and a resistor, said first circuit being connected across said capacitor, a second series circuit including a controllable discharge tube having an ionizable medium, a coil, a first mechanical switch, an electric light source for producing a light flash for a predetermined period of time and an impedance, said second circuit being connected across said capacitor, first and second rectifying tubes each having an anode, a cathode and a control grid, each of said cathodes being connected to one terminal of said impedance and each of said control grids being connected to the other terminal of said impedance to render said first and second tubes non-conducting during said predetermined period of time, a transformer having a primary winding connected to a source of alternating voltage and a secondary winding connected between the anodes of said first and second tubes, said secondary winding having a tap thereon, a relay having an energizing winding disposed to actuate said mechanical switch, said energizing winding being connected at one terminal to the one terminal of said impedance and a second mechanical switch connected between the other terminal of said energizing winding and the tap on said secondary winding whereby said first switch is actuated to de-energize said light source when said light source is energized for a period of time longer than said predetermined period of time.

References Cited in the file of this patent UNITED STATES PATENTS 

